Transducers convert signals from one domain to another. For example, some sensors are transducers that convert physical signals into electrical signals. On the other hand, some transducers convert electrical signals into physical signals. A common type of sensor is a pressure sensor that converts pressure differences and/or pressure changes into electrical signals. Pressure sensors have numerous applications including, for example, atmospheric pressure sensing, altitude sensing, and weather monitoring. Another common type of sensor is a microphone that converts acoustic signals into electrical signals.
Microelectromechanical systems (MEMS) based transducers include a family of transducers produced using micromachining techniques. MEMS, such as a MEMS pressure sensor or a MEMS microphone, gather information from the environment by measuring the change of physical state in the transducer and transferring the signal to be processed by the electronics, which are connected to the MEMS sensor. MEMS devices may be manufactured using micromachining fabrication techniques similar to those used for integrated circuits.
MEMS devices may be designed to function as oscillators, resonators, accelerometers, gyroscopes, pressure sensors, microphones, microspeakers, and/or micro-mirrors, for example. Many MEMS devices use capacitive sensing techniques for transducing the physical phenomenon into electrical signals. In such applications, the capacitance change in the sensor is converted to a voltage signal using interface circuits.
Microphones and microspeakers may also be implemented as capacitive MEMS devices that include deflectable membranes and rigid backplates. For a microphone, an acoustic signal as a pressure difference causes the membrane to deflect. Generally, the deflection of the membrane causes a change in distance between the membrane and the backplate, thereby changing the capacitance. Thus, the microphone measures the acoustic signal and generates an electrical signal. For a microspeaker, an electrical signal is applied between the backplate and the membrane at a certain frequency. The electrical signal causes the membrane to oscillate at the frequency of the applied electrical signal, which changes the distance between the backplate and the membrane. As the membrane oscillates, the deflections of the membrane cause local pressure changes in the surrounding medium and produce acoustic signals, i.e., sound waves.
In MEMS microphones or microspeakers, as well as in other MEMS devices that include deflectable structures with applied voltages for sensing or actuation, pull-in or collapse is a common issue. If a voltage is applied to the backplate and the membrane, there is a risk of sticking as the membrane and the backplate move closer together during deflection. This sticking of the two plates is often referred to as pull-in or collapse and may cause device failure in some cases. Collapse generally occurs because the attractive force caused by a voltage difference between the membrane and the backplate may increase quickly as the distance between the membrane and the backplate decreases.